Critical Mass, London, England, was started in 1994 and has been having regular monthly rides ever since.

Critical Mass is sometimes criticised for 'corking', preventing drivers from entering the stream of oncoming bicycles from side-turnings. The video shows a police car being corked but waiting patiently for the procession of bicycles to pass, which every driver should do. It also shows how taxis and motorbikes try to enter the Mass and create a hazard. Another criticism is that the ride prevents pedestrians crossing the road but the video clearly shows that they are often able to cross easily due to the slow speed of the Mass.

http://www.criticalmasslondon.org.uk/index.html